Horn mouth dimension.. how low will it go? - diyAudio
 Horn mouth dimension.. how low will it go?
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 2nd November 2010, 10:16 AM #1 diyAudio Member     Join Date: Jun 2008 Location: London/Bangkok Horn mouth dimension.. how low will it go? Hi guys, I'm rubbish with maths and I know many other factors have to be taken into account Q. if I built a rectangular front horn with a mouth approx 180cm x 60 cm, realistically what wd I get down to? The horn will be vertical, placed 15 cm above floor level.
 2nd November 2010, 10:22 AM #2 diyAudio Member     Join Date: Jun 2008 Location: London/Bangkok I think 180 x 60 gives an mouth area of 10800 cm2
 2nd November 2010, 12:10 PM #3 diyAudio Moderator R.I.P.   Join Date: Nov 2005 I guess it depends on what you are asking is it a bass horn, or a fronthorn waveguide with a bass horn, my guess is 35hz for a fronthorn, maybe 100-200hz and I would expect it to be slightly "less efficient", because of the dimensions and vertical design but its guessing __________________ sometimes we know very little, and sometimes we know too much
 2nd November 2010, 12:56 PM #4 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Hi, sitting on the floor away from the walls is a 2PI loading. Double the area to give the 4Pi equivalent. Now find the circle with the same area as that 4Pi equivalent. Now find the circumference of that equal area circle. That is the limit for maximum wavelength that the horn loads the driver. Any lower in frequency and ripples form, due to poor coupling. The ripples in the frequency response get worse the lower you ask the speaker to reproduce. The horn is considered truncated when it is shorter than a half wavelength of the lowest frequency you want to reproduce. Truncations down to 60% are often adopted in commercial speakers. This seems to be an acceptable compromise. The Flare rate of the horn also determines the lowest frequency that it can reproduce cleanly. This is all based on the old fashioned and somewhat empirical rules for horn design. Hornresp would do a better job of analysing your horn. If you want to get a feel for what it might do, then using: 2PiArea = 1.8 * 0.6 = 1.08m^2 4PiArea = 2 * 1.08 = 2.16m^2 Diameter of circle for equal area = sqrt(4Piarea *4 / Pi) = 1.65m Circumference of equal area circle = Sqrt(4*Pi*4Piarea) = 5.2m Lowest frequency for full horn loading = speed of sound / circumference =340/sqrt(4*Pi*4piarea) = 340 / [4 * 3.14159 * 2.16] = 65Hz. Minimum length for full horn loading = 5.2/2 = 2.6m __________________ regards Andrew T.
 3rd November 2010, 01:31 AM #5 diyAudio Member     Join Date: Jun 2008 Location: London/Bangkok Thanks Andrew, thats an awesome reply! The minimum length for the front horn loading is going to be hard to implement...! don't really want the speaker cabs to be more than a metre deep- how do the tannoy westminster's work? I could reduce the mouth area by say, a quarter( 135 x 45 cm) - too tired to do the maths(1.30am) but if its around 100 hz that will be fine.
 3rd November 2010, 10:48 AM #6 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders Hi, there are many pics of the Westminster showing the folding. I think one of Tannoy's brochures show the internals. The Westminster is truncated. From memory the horn is ~3.5m long. (if this is correct it could indicated a truncated 6m long horn) The mouth area, as a result of truncation is quite small. Presumably Tannoy experimented with many truncation ratios to come up with a commercially viable outcome. For £15,000 you would expect good bass performance. BTW, the Westminster is a 3horn design. The treble is front loaded compression driver. 1kHz to 20kHz The mid is front loaded cone. 250Hz to 1kHz The bass is back loaded cone folded. 18Hz to 250Hz __________________ regards Andrew T. Last edited by AndrewT; 3rd November 2010 at 10:53 AM.
 3rd November 2010, 02:33 PM #7 diyAudio Member     Join Date: Jun 2008 Location: London/Bangkok I think I should explain the design- I want to build a floorstander from corian at around 170-200 cm high using a vertical rectangular front horn- its a 2 way with a small full range unit / front horn and a 15-18" pro speaker firing downwards(BR) at the base of the cab into an omni type flare. The BR volume utilises the spare space in the cabinet around the front horn and the cabinet shape will a sonus faber type shape. I may be way over my head with this
 3rd November 2010, 02:58 PM #8 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders use hornresp __________________ regards Andrew T.
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